Conventionally, in longitudinal seam-welding, for manufacturing a welded steel pipe, the groove of an O-shaped plate (hereinafter called "pipe-blank"), formed into a cylindrical shape by a forming process such as the U-O process (abbreviation of the U-ing/O-ing process) and the bending roll process, as shown in the schematic side view given in FIG. 1 for example, it is the usual practice to employ an inside welding machine equipped with a welding torch 1 attached to the free end of a boom 2 having a length at least equal to that of a pipe-blank 3 to be welded, the fixed end of said boom 2 being fixed to a carriage 4; to insert said boom 2 into said pipe-blank 3 in advance; and, to longitudinally seam-weld said pipe-blank 3 from the inside along the groove with a consumable electrode 5 fed through said welding torch 1, while moving said boom 2 by said carriage 4, together with a cable 9 serving to supply welding current to said consumable electrode 5, in the withdrawal direction from said pipe-blank 4, i.e., in the welding direction as indicated by an arrow in the drawing. An inside welding machine equipped with two welding torches 1 and 1, leading and trailing, and two consumable electrodes 5 and 5 is represented in FIG. 1. However, an inside welding machine may have one welding torch and one consumable electrode, or it may be provided with more than two welding torches and more than two consumable electrodes.
In the aforementioned conventional welding method of a pipe-blank, when adopting a welding process comprising using direct electric current as the welding current with a consumable electrode as the anode, such as the reversepolarity GMA welding process (GMA welding process is the abbreviation of the gas metal arc welding which comprises carrying out welding while shielding a molten metal and a welding arc produced in the space between the base metal and the consumable electrode from open air with shielding gases such as an inert gas and a carbon dioxide gas), as shown in the partially enlarged schematic drawing given in FIG. 2, the welding arc 6 from the consumable electrode 5 is deflected toward the upstream side of the welding direction indicated by an arrow in the drawing, i.e., in the opposite direction to that of welding, and takes the form as if it is drawn in by the molten metal 7. When the welding arc 6 takes the form as if it is drawn in by the molten metal 7 as mentioned above, the plasma jet stream produced at the tip of the consumable electrode 5 is also deflected toward the molten metal 7 and acts on said molten metal 7 as a dynamic pressure. This pushes said molten metal 7 away toward the upstream side of the welding direction, i.e., in the opposite direction to that of welding. As a result, the space below the consumable electrode 5 becomes substantially dry without molten metal, thus impairing the affinity between molten droplets from the consumable electrode 5 and the base metal at the groove of the pipe-blank 3. Welding defects such as undercut of bead, humping bead and lack of fusion of base metal tend to easily occur. In addition, frequent occurrence of boiling and spattering in the molten metal 7 tends to result in a deteriorated appearance of the weld bead. When the welding arc 6 is deflected as described above, furthermore, the tip of the consumable electrode 5 is melted only one one side as shown in FIG. 2. Under such circumstances, the transfer mode of molten droplets from the consumable electrode 5 cannot be a desirable spray transfer, but takes an undesirable mixed form of globular transfer and short-circuit transfer. As a result, coarse spatters are splashed with a crackling shot-circuiting noise and are deposited on the weld bead surface, thus leading to a deteriorated appearance of the weld bead. What is worse, splashed spatters are deposited on the opening at the tip of the shielding nozzle (not shown) of the welding torch to disturb the gas shield and entangle the air. In this case, it may practically be impossible to carry out welding.
The above-mentioned deflection of the welding arc toward the upstream side of the welding direction, i.e., in the opposite direction to that of welding, and the resulting irregular weld bead and welding defects are not limited only in the case of the conventional GMA welding process, but occur also in the case of the submerged-arc welding process using direct electric current as the welding current. In both cases, it has been difficult to obtain a beautiful and sound weld free from welding defects.
With a view to making basic considerations on the irregular weld bead and welding defects such as undercut of the bead, humping bead, lack of fusion of base metal and spattering, observed in longitudinally seam-welding a pipe-blank for welded steel pipe from inside along a groove by the conventional welding process as mentioned above, and to clarifying causes thereof, the inventors have made intensive studies. As a result, it has been found that the occurrence of said irregular weld bead and welding defects is attributable to the deflection of a plasma jet stream toward a molten metal caused by the deflection of a welding arc, and also that said deflection of the welding arc is brought about by a line of magnetic force produced by the direct electric current for welding flowing through a cable introduced into the pipe-blank.
More specifically, for instance, in longitudinally seam-welding a pipe-blank for welded steel pipe from the inside along a groove by the conventional reverse-polarity GMA welding process comprising using direct electric current as the welding current with a consumable electrode as the anode, direct electric current for welding flows, as shown in the partial cutaway schematic side view given in FIG. 3, in the opposite direction to that of welding indicated by an arrow in the drawing, through a cabtyre cable 9 introduced into the pipe-blank 3.
Therefore, a line of magnetic force 10, which is clockwise as viewed from the right-hand side of the drawing, is produced by said direct electric current, and a strong magnetic field is formed around said cable 9. As a result, said pipe-blank 3 is strongly magnetized in the same clockwise direction 11 as that of the line of magnetic force 10 by said magnetic field, thus causing leakage of a strong line of magnetic force from the groove of said pipe-blank 3, and a strong magnetic field is formed at said groove. When welding a groove where such a strong magnetic field is formed, the welding arc from a consumable electrode 5, which is a flow of charged corpuscle, is deflected by said strong magnetic field at the groove. This phenomenon is called the magnetic arc blow of the welding arc.
The relation between the direction of the electric current flowing through a welding arc, the magnetizing direction at the groove of a pipe-blank and the direction of the force acting on said welding arc at said groove is illustrated in the vector diagrams of FIGS. 4A and 4B. In these drawings, A is the direction of the electric current flowing in the welding arc 6 through the consumable electrode 5; B is the magnetizing direction at the groove of the pipe-blank 3; C is the welding direction; and F is the direction of the force acting on said welding arc 6. As shown in FIGS. 4A and 4B, the direction F of the force acting on the welding arc 6 is the same as that of the direct electric current for welding flowing through the cable 9 (refer to FIG. 3) and is opposite to the welding direction C. As described above with reference to FIG. 2, therefore, the welding arc 6 is deflected toward the upstream side of the welding direction C, i.e., in the opposite direction to that of welding C, thus resulting in such irregular weld bead and welding defects as undercut of the bead, humping bead, lack of fusion of base metal and spattering.
The above-mentioned magnetic arc blow of the welding arc is widely observed when applying the arc welding process using direct electric current as the welding current.
The inventors have heretofore proposed, based on the aforementioned finding, a welding method which comprises, in longitudinally seam-welding a pipe-blank for welded steel pipe from the inside along a groove by the reverse-polarity GMA welding process comprising using direct electric current supplied to a consumable electrode through a cable as the welding current with said consumable electrode as the anode, setting the direction of said direct electric current for welding flowing through said cable introduced into said pipe-blank in the same direction as that of welding, thereby preventing magnetic arc blow of the welding arc from said consumable electrode toward the upstream side of the welding direction, i.e., in the opposite direction to that of welding (hereinafter called"the first prior art") in Japanese Patent Application No. 31,392/76.
In Japanese Patent Publication No. 19,696/69, also, an apparatus for preventing magnetic arc blow of the welding arc is disclosed, which comprises, in longitudinally seam-welding a workpiece by the arc welding process comprising using direct electric current as the welding current, placing a magnetic body at a position closest to a welding electrode, in contact with or as close as possible to said workpiece, across a groove of said workpiece (hereinafter called "the second prior art"). According to said second prior art, a magnetic flux produced by the direct electric current for welding is almost totally focussed into said magnetic body, and the welding arc is not affected by the magnetic flux, whereby the magnetic arc blow of the welding arc is prevented.
According to the above-mentioned first and second prior arts, in longitudinally seam-welding a pipeblank from the inside along a groove, it is possible to avoid an adverse effect of magnetic arc blow of the welding arc, and hence to obtain a beautiful and sound weld free from defects.
However, even in the case where, after a pipeblank is longitudinally seam-welded from inside along a groove by said first and second prior arts, said pipeblank is longitudinally seam-welded from the outside along said groove, a magnetic arc blow of the welding arc as mentioned above often occurs, thus causing boiling and spattering of molten metal. The appearance of the resulting weld bead tends to be deteriorated, and moreover, this leads to another problem of easy occurrence of such welding defects as undercut of the bead and lack of fusion of base metal.
This is considered attributable to the fact that, while the line of magnetic force 10 shown in FIG. 3 disappears immediately after the completion of longitudinal seam-welding of the pipe-blank from the inside along the groove, said pipe-blank itself has in general a considerable coercive force, and retains therefore a considerable residual magnetism for a long period of time. Consequently, in longitudinally seam-welding the pipe-blank from outside along the groove, which is previously longitudinally seamwelded from the inside, said residual magnetism causes a magnetic arc blow of the welding arc.
The aforementioned adverse effect of residual magnetism not only occurs in the case where a pipe-blank is longitudinally seam-welded on both sides from the inside and outside along a groove, but also is known to occur in the case where a pipeline is formed by successively butt-welding welded steel pipes, in the axial direction thereof, manufactured by longitudinally seam-welding pipe-blanks on one side from the inside along a groove.